1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems use a network of base stations to provide wireless connectivity to one or more mobile units. In some cases, the mobile units may initiate wireless communication with one or more base stations in the network, e.g., when the user of the mobile unit would like to initiate a voice or data call. Alternatively, the network may initiate the wireless communication link with the mobile unit. For example, in conventional hierarchical wireless communications, a server transmits voice and/or data destined for a target mobile unit to a central element such as such as a Radio Network Controller (RNC). The RNC may then transmit paging messages to the target mobile unit via one or more base stations. The target mobile unit may establish a wireless link to one or more of the base stations in response to receiving the page from the wireless communication system. A radio resource management function within the RNC receives the voice and/or data and coordinates the radio and time resources used by the set of base stations to transmit the information to the target mobile unit. The radio resource management function can perform fine grain control to allocate and release resources for broadcast transmission over a set of base stations.
Secure communications in a conventional hierarchical system, such as a CDMA system, are established based on secret information (e.g., an Authentication Key) known only to the mobile unit and a secure entity in the network. The HLR/AuC and the mobile unit may derive shared secret data (SSD) from the Authentication Key (AK), e.g., using the CAVE algorithm. The AK is a 64-bit primary secret key known only to the mobile station and the HLR/AuC. This key is never shared with roaming partners. The AK may be used to generate the SSD, which is a 128-bit secondary key that can be calculated using the CAVE algorithm and can be shared with roaming partners. During authentication, the HLR/AuC and the mobile unit both calculate an Authentication Response separately and independently using shared inputs such as SSD, electronic serial number (ESN), Mobile Identity Number (MIN), and a shared Random Number (RAND). If the independently calculated results match up, then authentication is approved and the mobile unit is allowed to register with the network.
The AK or SSD can be used to authenticate mobile units that are registered in the network. For example, a base station may periodically generate a random number (RAND) and broadcast the RAND. Mobile units that receive the broadcast RAND compute an authentication algorithm output (AUTH) using the inputs including the RAND and the AK or SSD. The AUTH and the associated RAND (or selected portions of the RAND) are sometimes referred to as a pair. The mobile unit may then transmit the AUTH/RAND pair to the base station, which may then pass this information through the network on to the HLR/AuC. The HLR/AuC uses the authentication algorithm, the stored value of the AK or SSD, other data corresponding to each mobile unit, and the RAND to calculate the expected value of AUTH. If this value matches the value transmitted by the mobile unit, the mobile unit is authenticated. The base station frequently changes the value of RAND to ensure that the AUTH value is fresh and to reduce the possibility that previously generated AUTH/RAND results may be captured by monitoring the air interface and replayed by a fraudulent mobile unit or mobile unit emulator. This technique is considered reasonably reliable, at least in part because base stations are typically secure devices that are under the control of wireless communication providers.
One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of access points, such as base station routers, that implement distributed communication network functionality. For example, each base station router may combine RNC and/or PDSN functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the RNC and PDSN of hierarchical networks may be reduced or removed.
At least in part because of the reduced cost and complexity of deploying a base station router, base station routers may be deployed in locations that are impractical for conventional base stations. For example, a base station router may be deployed in a residence or building to provide wireless connectivity to the occupants of the residents of the building. Base station routers deployed in a residence are typically referred to as home base station routers or femtocells because they are intended to provide wireless connectivity to a much smaller area (e.g., a femtocell) that encompasses a residence. However, the functionality in a femtocell is typically quite similar to the functionality implemented in a conventional base station router that is intended to provide wireless connectivity to a macro-cell that may cover an area of approximately a few square kilometers. One important difference between a femtocell and a conventional base station router is that home base station routers are designed to be inexpensive plug-and-play devices that can be purchased off-the-shelf and easily installed by a lay person.
Femtocells do not typically include expensive security chips for storing information that can be used to establish secure communications between the femtocell and mobile units. Furthermore, femtocells are intended to be deployed in unsecured locations, such as a person's home or place of business. Consequently, femtocells are not considered trusted locations for storing secret keys or other information that may be used to authenticate mobile units. A femtocell may therefore be modified to fraudulently represent a mobile unit if femtocells are configured to generate the random numbers RAND used to authenticate mobile units. For example, an illegitimate femtocell may intercept a valid AUTH/RAND pair transmitted between a legitimate mobile unit and a legitimate base station. The illegitimate femtocell may then emulate the legitimate mobile unit using the intercepted AUTH/RAND pair. Since the femtocell is responsible for generating RAND values, the network cannot determine whether or not the AUTH/RAND pair transmitted by the illegitimate femtocell corresponds to a fresh value of RAND.